The aim of this project is to measure the fluorescence quantum yield, fluorescence lifetime, triplet quantum yield and triplet lifetime for fifteen photosensitizers, and correlate these measurements with the comparative in vitro photoinactivation efficiency of these fifteen photosensitizers in PTK2 rat kangaroo epithelial kidney cells. The fifteen photosensitizers are: phthalocyanine (Pc), its di- and tetrasulfonate (PcDs,PcTs); zinc metallated complexes of Pc, PcDs, PcTs; chloroaluminum phthalocyanine and its tetrasulfonate (CIAIPc, CIAIPcTs); napthalocyanine (NPc), ZnNpc, methyl pheophorbide a (Mppb); mono-L- aspartyl chlorin E6 (MACE); tin etiopurpurin (SnEt); hematoporphyrin-IX and Photofrin (PII), a concentrated form of HpD. Photophysical measurements will take place in Dimethylformamide (DMF) and photocytotoxicity studies will take place in phosphate buffered saline (PBS), where possible, or in PBS- cremophor for the chlorin derivatives. Photosensitizers will be introduced to PTK2 rat kangaroo epithelial cells for twenty-four hours, exposed to light at the appropriate wavelength from a Nd-YAG laser, and the photokill efficiency of each complex assessed utilizing the trypan blue test for cell viability. This experiment seeks to provide a model for predicting the photokill efficiency of photosensitizers based on their photophysical parameters. This study represents the first comprehensive study of the photo physical behavior and the photocytotoxicity efficiency of a wide variety of photosensitizers, including "new" and "old" classes of photosensitizers- phthalocyanine and its derivatives, chlorophyll and chlorin derivatives- and Photofrin II. The hypothesis is that the best photosensitizers are those which possess a long-lived triplet state in high quantum yield when the oxygen concentration remains constant.